A common method for creating an integrated circuit (IC) involves an IC designer receiving a behavioral circuit model describing the desired functionality of the IC as well as a library of standard circuit cells. In many cases, the designer may operate a computer-aided design (CAD) system and may use the library of standard cells to build the IC based upon the behavioral circuit.
First, the designer determines which particular cells are needed for the IC. These cells represent actual physical circuits in the IC, and may include fundamental logic gates such as OR, NAND, NOR, AND, XOR, inverters, and other logical cells with an array of logic gate sizes. Then the designer reviews the behavioral circuit, either manually or automatically, to identify the particular cells required. Once the required logical cells are determined, the designer creates or retrieves a base logical cell structure for each cell type. Each base logical cell structure contains logical representations of transistors and other cell elements required to perform the particular logic functions of the cell. The base logical cell structure includes two-dimensional geometric data representing expected dimensions of the transistors and structures.
Thereafter, the designer couples the selected cells together via a graphical interface to create physical cell structures. In some cases, the designer also determines the physical placement of transistors and other cell elements within each logical cell structure, and identifies the conductive routing between the transistors and elements to form the required logic gates. Today at least portion of these placement and routing operations, collectively referred to as “layout operations,” may be performed automatically via the CAD system.
The inventor hereof has recognized, however, that it is very difficult and time-consuming to create high quality layouts for complex standard cells, such as flip-flops, either manually or by automated layout synthesis tools. Although automated tools can create such layouts much quicker than manual processes, the quality of automated layouts is typically lower than manually created layouts, especially in terms of the silicon area used.